feat: add sliceFrom to ListCompositeTreeViewDU

packages/ssz/src/viewDU/listComposite.ts

@@ -1,4 +1,4 @@
-import {Node, treeZeroAfterIndex} from "@chainsafe/persistent-merkle-tree";
+import {Node, subtreeFillToContents, treeZeroAfterIndex, zeroNode} from "@chainsafe/persistent-merkle-tree";
 import {ByteViews, ValueOf} from "../type/abstract";
 import {CompositeType, CompositeView, CompositeViewDU} from "../type/composite";
 import {ListCompositeType} from "../view/listComposite";
@@ -66,6 +66,43 @@ export class ListCompositeTreeViewDU<
     return this.type.getViewDU(newRootNode) as this;
   }
 
+  /**
+   * Returns a new ListCompositeTreeViewDU instance with the values from `index` to the end of list
+   *
+   * ```ts
+   * const nodes = getChunkNodes()
+   * return listFromChunkNodes(node.clice(index))
+   * ```
+   *
+   * Note: If index === n, returns an empty list of length 0
+   *
+   */
+  sliceFrom(index: number): this {
+    // Commit before getting rootNode to ensure all pending data is in the rootNode
+    this.commit();
+
+    // If slicing from 0, no slicing is necesary
+    if (index <= 0) {
+      return this;
+    }
+
+    let newChunksNode;
+    let newLength;
+
+    if (index >= this.nodes.length) {
+      newChunksNode = zeroNode(this.type.chunkDepth);
+      newLength = 0;
+    } else {
+      const nodes = this.nodes.slice(index);
+      newChunksNode = subtreeFillToContents(nodes, this.type.chunkDepth);
+      newLength = nodes.length;
+    }
+
+    const newRootNode = this.type.tree_setChunksNode(this._rootNode, newChunksNode, newLength);
+
+    return this.type.getViewDU(newRootNode) as this;
+  }
+
   /**
    * Same method to `type/listComposite.ts` leveraging cached nodes.
    */

packages/ssz/test/unit/byType/listComposite/tree.test.ts

@@ -194,3 +194,34 @@ describe("ListCompositeType.sliceTo", () => {
     }
   });
 });
+
+describe("ListCompositeType.sliceFrom", () => {
+  it("Slice List at multiple length", () => {
+    const listType = new ListCompositeType(ssz.Root, 1024);
+    const listView = listType.defaultViewDU();
+    const listRoots: string[] = [];
+    const listSerialized: string[] = [];
+
+    for (let i = 0; i < 16; i++) {
+      listView.push(Buffer.alloc(32, 0xf + i));
+    }
+
+    for (let i = 0; i < 16; i++) {
+      const currentListView = listType.defaultViewDU();
+
+      for (let j = i; j < 16; j++) {
+        currentListView.push(Buffer.alloc(32, 0xf + j));
+        listSerialized[i] = toHexString(currentListView.serialize());
+        listRoots[i] = toHexString(currentListView.hashTreeRoot());
+      }
+    }
+
+    for (let i = -1; i < 16; i++) {
+      const index = i !== -1 ? i : 0;
+      const listSlice = listView.sliceFrom(index);
+      expect(listSlice.length).to.equal(16 - index, `Wrong length at .sliceFrom(${i})`);
+      expect(toHexString(listSlice.serialize())).equals(listSerialized[index], `Wrong serialize at .sliceFrom(${i})`);
+      expect(toHexString(listSlice.hashTreeRoot())).equals(listRoots[index], `Wrong root at .sliceFrom(${i})`);
+    }
+  });
+});